Fusion down-draw is a leading technology developed by Corning Incorporated, Corning, N.Y., U.S.A. for making thin, precision glass sheets suitable for use as liquid crystal display (LCD) glass substrates and in other opto-electronic devices. This process is schematically illustrated in FIG. 1. A stream of molten glass is introduced into a forming trough 103 called isopipe with end-caps 105 at both ends and having two side surfaces converging at a line called root 109 via an inlet pipe 101 coupled to the trough of the isopipe. The glass melt is allowed to flow over both top surfaces of the trough side walls of the isopipe called weirs, down along both side surfaces of the isopipe as two molten glass ribbons 107, and then join and fuse at the root 109 to form a single glass ribbon 111, which is then drawn down in the direction 113 and cooled below the root to form the glass sheet with desired dimension. In the zone below the root, the glass ribbon travels substantially vertically downward while being drawn and cooled from a viscous state, to visco-elastic and eventually to substantially elastic. The elastic glass ribbon is then cut into individual glass sheets, subjected to further finishing such as edge rounding and polishing, and then packaged and shipped to LCD panel makers for use as TFT or color filter substrates. Cutting of the glass ribbon at below the isopipe typically involves the scoring of the ribbon surface, followed by bending along the score-line, whereby discrete glass sheets are separated from the ribbon and then transferred to subsequent steps.
One of the advantages of the fusion down-draw process for making glass sheets is that the surface quality of the glass sheets is high because the quality areas thereof were formed in an atmosphere and never touched a solid material such as the forming equipment. This process has been used successfully for making glass sheets having a width as large as 3000 mm and a thickness of about 0.6 mm.
The size of LCDs for the consumer electronics market has grown steadily in the past decade, and along with a corresponding demand for higher image quality. These have fueled the demand of large-width glass substrates and posed increasingly more stringent requirements for glass sheet quality, such as edge warp and waviness, sheet warp, surface waviness and roughness, thickness uniformity, mura, as well as stress. In addition, consumers have demonstrated interest in lighter electronics, which call for thinner glass substrates having a thickness of 500 μm, 400 μm, 300 μm or even lower.
Making large-size and/or thin glass sheets using the fusion down-draw is no easy undertaking, requiring the use of new generations of isopipes having significantly larger length. Over the years, experts such as the present inventors have gained insights into the many process parameters that can impact the process stability during the forming process. Particularly, it was found that the formation of a stable, thin glass ribbon over the large width of a large-generation isopipe may not be achieved reliably in a relatively short period of time within an economical process widow having a desirable yield without proper glass melt flow velocity which can be quite different from the flow velocity for smaller isopipes.
Therefore, there remains a need of an apparatus and method for making glass sheets having a large size. The present invention satisfies this and other needs.